Gandalf and Bilbo are different in many ways.
Gandalf is a wizard. There are very few wizards in Tolkien's Middle Earth (the setting of his stories) and Gandalf is one of two that have a significant effect there; the other is Sauruman the white wizard; he is the leader of the wizards in Middle Earth. Gandalf is known as Gandalf the grey, friend of elves and dwarves. Tolkien created Gandalf in the tradition of the wizard Merlin from Sir Thomas Malory's Le Morte D'Arthur; he is a subtle but powerful manipulator of world affairs.
Bilbo is a hobbit or a halfling. Bilbo was selected by Gandalf to be the “burglar” for Thorin Oakenshield and his party of dwarves that were making the quest to reclaim Lonely Mountain from the dragon, Smaug who took it from Thorin and his ancestors. Bilbo is an unassuming character that becomes the hero of the story through his honest integrity and spirit.